A comparison was made of the discriminating power of the E-Test and conventional methods to distinguish different bacterial strains harboring plasmid-mediated extended-spectrum beta-lactamases. The E-Test provided quantitative data comparable to those of a broad range of concentrations in the agar dilution method, but proved to be far superior to the disk diffusion test or the broth microdilution semiautomatic PASCO system for the detection and hence for the epidemiologic surveillance of this type of resistant strain.